Among respiratory viruses influenza A virus (IAV) is particularly prone to cause bacterial superinfections. These infections are the major cause of morbidity and mortality during IAV epidemics. There is compelling evidence that an acquired IAV-induced defect in phagocyte function is an important cause of bacterial superinfection. Binding of the IAV hemagglutinin to sialic acid-bearing receptor sites on the neutrophil mediates depression of cell function. Preliminary studies indicate that leukosialin, CD45, sialyl-Le/x, and gangliosides are among the sites bound. We propose to confirm that IAV binds to these sites, establish specific sialyl-Le/x bearing proteins which are bound, determine how the various binding proteins are perturbed (i.e. internalized, capped) by IAV, and establish which sites are most important quantitatively and functionally. Our preliminary data are compatible with the hypothesis that a single glycoprotein receptor mediates deactivation. The definitive test of this theory will involve eliminating expression of specific neutrophil glycoproteins in HL60 cells using recombinant (i.e. antisense RNA or gene targeting) techniques. Identification of functionally important glycoprotein receptors for IAV is unlikely to provide a full explanation for deactivation. We will also need to establish how IAV particles perturb these receptors such that deactivation occurs. Pre-incubation of IAV particles with pulmonary surfactant protein D (SP-D) reduces the ability of IAV to cause neutrophil dysfunction, while markedly enhancing the ability of IAV to stimulate a respiratory burst response. However, IAV complexed with SP-D binds to neutrophils via the same mechanism as unopsonized IAV (i.e. via attachment to sialic-acid bearing neutrophil membrane components). Using recombinant, wild type and mutant, SP-D preparations we will determine the mechanisms through which "opsonization" of IAV particles with SP-D alters the functional outcome of IAVs interaction with neutrophils. We will characterize the aggregates formed after incubation of IAV with SP-D (by light scattering, fluorescent and EM techniques) and determine how these aggregates bind to neutrophils [by identifying specific binding sites and demonstrating how these binding sites are perturbed (e.g. capped, internalized) in intact cells]. Our working hypothesis is that SP-D's protective effect results not from changing which neutrophil receptors bind IAV but rather from SP-D's ability to change the nature of the IAV particle.